(1) Field of the Invention
The present invention relates to power integrated circuit devices and, more particularly, to high voltage diodes formed in power integrated circuits.
(2) Description of Related Art
It is often desirable, if not necessary, to form high-voltage diodes integrally with, i.e., on the same substrate as, power integrated circuits (PICs). For this purpose, a Lateral PNP transistor is often formed integrally with a PIC and interconnected to function as a high-voltage diode.
Such a diode-connected lateral PNP transistor is shown in FIG. 1. Diode-connected transistor 10 generally includes a substrate 12 of a first conductivity type, such as, for example, p-type. An isolation region 14 of a second conductivity type, such as, for example, n-type, is formed in a portion of an epitaxial layer that is grown on substrate 12. A high voltage (HV) well 16 (or epitaxial layer) of the second conductivity type is formed above the isolation region 14, and an emitter well 18 of the first conductivity type is formed in the HV well 16. A collector well 20 of the first conductivity type is formed in the HV well 16, and a base well 22 of the second conductivity type is formed in HV well 16 and spaced apart from collector well 20. Field oxide isolation layers 24 and 26 are formed on the surface of HV well 16, with field oxide 24 disposed between emitter well 18 and collector well 20 and field oxide 26 disposed between collector well 20 and base well 22. Drift region 28, to support high reverse-bias voltage, includes the portion of HV well 16 between emitter well 18 and collector well 20, i.e., the portion of HV well 16 underlying field oxide 24. A lateral transistor 30 is formed between emitter well 18, collector well 20 and base well 22. Collector well 20 and base well 22 are interconnected to form the cathode, and the emitter well 18 forms the anode, of the diode-connected transistor 10.
Generally, a high-voltage diode desirably has a low on-state resistance (low forward voltage drop), fast switching speed, low parasitic substrate current and a high reverse breakdown voltage. However, diode-connected transistors are somewhat limited in respect to the aforementioned desired characteristics. More particularly, the reverse breakdown voltage of such a diode is determined in large part by the length of drift region 28, i.e., longer drift regions provide higher reverse breakdown voltages. For example, in a 0.35 micron technology device, a drift region of approximately 6 microns in length provides a reverse breakdown voltage of only 32 Volts due to shallow junctions in the advanced technology device. Thus it is seen that producing devices with high reverse breakdown voltages, and therefore relatively long drift regions, deeper junctions and increased mask count, undesirably consumes large amounts of real estate on the integrated circuit substrate, increases costs and increases the forward bias voltage drop due to the high on-state resistance of the diode on the integrated circuit substrate. Measures to more evenly distribute the electrical field, such as, for example, polysilicon field plates, provide only moderate improvement in reverse breakdown voltage for a given drift length with shallow junctions.
Such diode-connected transistors also generally have an undesirably low current gain (beta) between the emitter/anode and collector/cathode. The low current gain is primarily due to the relatively long drift region that separates the emitter and collector regions. When the diode-connected transistor is forward-biased, a vertical parasitic transistor existing between the emitter/anode region, drift region, and substrate is also forward biased. This vertical parasitic transistor is represented in FIG. 1 by transistor 34, which has HV NWELL 16 and NISO 14 as a base, emitter well 18 as an emitter, and substrate 12 as a collector. The vertical parasitic transistor 34 conducts a parasitic substrate leakage current from the emitter well 18 (emitter/anode) to substrate 12 (collector). Due to the low current gain of the diode-connected transistor 12 (or the lateral transistor), the substrate leakage current conducted by the vertical parasitic transistor 34 is typically of an appreciable magnitude relative to the current carried by diode-connected lateral transistor 12. Under some circumstances, the substrate leakage current may dominate the operation of the diode, such as, for example, in a device having a large drift length and a low dopant concentration in the isolation region.
FIG. 2 illustrates another embodiment of a diode-connected lateral transistor 10A in which a heavily-doped buried layer 14A of the second conductivity type, used as an isolation layer, with an overlying deep HV well or epitaxial layer 16A of the second conductivity type are used to reduce the leakage current carried by the vertical parasitic transistor 34A. However, the heavily-doped buried layer 14A and deep HV well or epitaxial layer 16A decrease the current gain of the diode-connected lateral transistor and reduce the reverse breakdown voltage of the diode, especially in deep sub-micron PIC technology. Therefore, in devices having a heavily-doped buried layer with an overlying epitaxial layer or HV well, a drift region of increased length is required to provide a given reverse breakdown voltage. As integrated circuit designers and manufacturers strive to reduce overall device size and thereby increase circuit density on integrated circuit substrates, increasing the drift length and/or depth of the HV well or epitaxial layer, which is also normally used as the drift region for drivers, such as LDMOS, is an undesirable approach to increasing reverse breakdown voltage.
Therefore, what is needed in the art is a diode formed integrally on the same substrate with an advanced PIC and which achieves a given level of protection against reverse breakdown and yet has a relatively small/short drift region and, thus, a reduced device size.
Furthermore, what is needed in the art is a diode formed integrally with and on the same substrate as an advanced PIC and which achieves a given level of protection against reverse breakdown and yet has a relatively small/short drift region and, thus, a reduced forward voltage drop.
Moreover, what is needed in the art is a method of fabricating a diode integrally with and on the same substrate as an advanced PIC and which achieves a given level of protection against reverse breakdown with a relatively small/short drift region and a reduced parasitic substrate leakage current.